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Real Climate published a guest commentary today by Cecilia Bitz, one of the authors on the paper suggesting that the Arctic sea ice may disappear by 2040. 1-12-07 Real Climate Post Arctic Sea Ice in the 21st Century Bitz answers a number of the questions that she was asked by the media in greater detail than the media was able to report. She offers hope that we haven't reached the point of no return -- especially if we can stabilize emissions at 2000 levels by 2020.

Here is a part of the post -- but be sure to review the full post and comments:

In our paper,
we examined the September Arctic sea ice cover in the 20th and 21st
centuries in climate models, and found occasional decades of very rapid
retreat. The most extreme case was a decrease from 6 to 2 million
square kilometers in a decade (see Fig 1). This is about 4 times faster
than the decline that has been observed in the past decade.

Figure 1: (a) Northern Hemisphere sea ice extent in September
from one integration of the Community Climate System Model version 3
(CCSM3) with observations from satellite era shown in red. The light
blue line is a 5-yr running mean. The three lower panels show the
September ice concentration (ice floes are separated by open water) in
three select decades.

It is common practice to run climate
models multiple times with slight variations to the initial conditions.
Because the system is chaotic, the natural variability in each run is
random and uncorrelated from one run to the next. When an ensemble of
runs is averaged, the natural variability is reduced in the ensemble
mean, and it is easier to detect a significant trend.

An
ensemble of runs offers an opportunity to evaluate rare events too,
such as extreme sea ice decay. We were in search of evidence for "tipping points",
which several authors have speculated might exist in sea ice.
RealClimate places sea ice in the category of systems with "known
unknowns" with regard to tipping points. This means we know there are
thresholds involving sea ice (e.g., it can cease to exist), but we
don't know when or if the climate will arrive at one.

Only one
of seven ensemble members had an event as extreme as quoted above, and
it resulted in near ice-free conditions for September by 2040 (see Fig
1d). (The sea ice grows back at least for some portion of winter for
the duration of the 21st century.) However, every ensemble member had
an event 5 years or longer at some time in the 21st century when the
sea ice retreat was about 3 times faster than the observed retreat
since 2001 (see Fig 2). These ensemble members took about 5--10 years
longer to become nearly ice-free in September than the most extreme
case.

As illustrated in Fig 1, the sea ice retreat accelerates
during the 21st century as the ice decays and more sunlight is absorbed
by the ocean (the positive ice-albedo feedback). Increasing ocean heat
transport under the sea ice adds to the melt back. The retreat appears
abrupt when natural variability in the ocean heat transport into the
Arctic Ocean is anomalously high. We did not find clear evidence of a
threshold, which can be difficult to identify given the variability and
complexity of the climate system. Therefore we can neither verify or
rule-out the existence of a tipping point. Regardless, the rapid
declines seen in our runs are a serious concern.

Figure 2: Northern Hemisphere sea ice extent in September for
all seven integration of the CCSM3 with observations from satellite era
shown in black.

Most common questions asked by journalists

1) How does our model compare with the trend in the observed record?

The
trends in the seven ensemble members for 1979-2006 span the trend in
the observations: Some members retreat a little faster and some a
little slower, as expected from the random natural variability in the
runs (see Fig 2). The model also reproduces the mean and variance of
the observations with good fidelity.

2) Other scientists are
predicting an ice-free Arctic in September by the year 2060-2080, why
is this model predicting it 20-40 years sooner?

First consider
estimates based on extrapolation from the observational record. I've
heard these numbers quoted in the media, but I have not seen a
reference to a scientific paper that discusses the analysis in any
detail. Figures 2 and 3 illustrate the danger of making an estimate of
the future from the observational period. The future trend is not
linear, the observational record is too short and the ice-free time is
too far in the future to trust extrapolation. If one carries out such
an exercise anyway, extrapolation from a linear fit to 1979--2006 gives
a zero intersect (indicating the first ice-free year in the future) at
about 2110 (see Fig 3). If instead one uses just the last decade, the
extrapolation gives 2060. Both estimates are questionable, and so
instead we turn to climate models.

Figure 3: Extrapolating into the future from the observational record.

3) Is sea ice in our model retreating faster than in other models?

Figure
4 shows September ice retreat in 16 models that were archived for the
IPCC AR4. The most extreme predictions are from models that have too
much or too little sea ice extent compared to observations, so it is
important for a model to produce the correct sea ice coverage in the
past. Some of the spread is expected from natural variability, but much
depends on differing model sensitivity relating to the representation
of sea ice, heat transport by the ocean, and cloud cover.
It is not possible to identify the most accurate model prediction,
although I think it is safe to rule out some of the outliers owing to
their poor match to the observations.

About half of the models
become ice-free in September during the 21st century. I included one
ensemble member from our model, CCSM3, which is in the middle of the
pack until about 2020. Our model run retreats faster than most after
about 2020, but it isn't radically different.

There is
considerable uncertainty in future model projections, and Figs 2 and 4
illustrate why it would be better not to focus too much on the year
2040, which to our dismay was highly publicized. The more important
message from models is that all but a few outliers predict enourmous
sea ice retreat this century. At least a few respectable models predict
a nearly ice-free Arctic by midcentury, with a retreat that may be
punctuated by rapid events.

Figure 4: Northern Hemisphere sea ice extent in September from
model integrations submitted to the IPCC AR4 with observations from
satellite era shown in black.

4) Is it too late to save the sea ice?

The future emissions scenario discussed here is one that assumes modest increases
in emissions. If humans can reduce the rise in emissions compared to
this, then sea ice retreat would be slower and rapid events would be
rarer, according to the IPCC AR4 models.

5) Have we crossed a tipping point?

I
don't think we have yet. If we fix the greenhouse gas and aerosol
levels at year 2000 values and run the model into the 21st century, the
sea ice retreats for only another decade or two and then levels off
(some of the ensemble members even recover a little bit). So according
to our model, the sea ice does not appear to have passed a threshold
yet. We have not done an exhaustive study of any years beyond today, so
unfortunately we cannot say with certainty that no tipping points
exist. The bottom-line: The retreat can be surprisingly rapid even
without clear evidence of a tipping point.